1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method for modifying image data expressing gray-scale values of pixels in an image according to variations in the gray-scale values at each pixel, the gray-scale values corresponding to voltages applied to a liquid crystal in a liquid crystal panel. The invention also relates to an image coding device and image coding method using this image processing apparatus and method.
2. Description of the Related Art
Liquid crystal panels are thin and lightweight, so they are widely used in display apparatus such as the display units of television receivers, computers, and mobile information terminals. However, they have the drawback of being incapable of dealing with rapidly changing moving pictures, because after application of a driving voltage, it takes some time for the desired transmittance to be reached. To solve this problem, a driving method that applies an excess voltage to the liquid crystal when the gray-scale value changes from frame to frame, so that the liquid crystal reaches the desired transmittance within one frame, has been adopted (patent document 1). More specifically, the image data of the current frame are compared pixel by pixel with the image data one frame before, and when there is a change in the gray-scale value, a correction corresponding to the change is added to the image data of the current frame. When the gray-scale values increases in comparison with the preceding frame, a driving voltage higher than the normal driving voltage is thereby applied to the liquid crystal panel; when the gray-scale value decreases, a driving voltage lower than the normal driving voltage is applied.
To practice the above method, it is necessary to have a frame memory from which to output the image data of the preceding frame. With the increasing numbers of pixels displayed on today's large liquid crystal panels, it becomes necessary to have an increasingly large frame memory. As the number of pixels increases, the amount of data that must be written into and read from the frame memory within a given time (within one frame interval, for example) also increases, so the frequency of the clock that controls the reading and writing of data and the data transfer rate must be increased. The increased size and transfer rate of the frame memory drive up the cost of the liquid crystal display apparatus.
To solve this problem, the image processing circuit for driving a liquid crystal described in patent document 2 reduces the size of the frame memory by encoding the image data before storing the image data in the frame memory. By correcting the image data on the basis of a comparison between decoded image data for the current frame obtained by decoding the encoded image data and decoded image data for the preceding frame obtained by delaying the encoded image data for one frame interval before decoding, it can also avoid the unnecessary application of excessive voltages associated with encoding and decoding errors when a still image is input.
Patent document 1: Japanese Patent No. 2616652
Patent document 2: Japanese Patent Application Publication No. 2004-163842
In the image processing circuit for driving a liquid crystal described in patent document 2, however, encoding is carried out by a block coding scheme that attempts to maintain a constant amount of quantized image data in the encoded image data, regardless of the type of input image, so when the coding compression ratio is increased to reduce the amount of coded data, the error due to coding and decoding increases, which has a major effect on the corrected image data. The problem accordingly arises that when the coding compression ratio is increased to reduce the amount of coded data, unnecessary excessive voltages are applied to the liquid crystal.